VariableSizedWrapGrid
As far as I already said, this class is almost the same as the WPF WrapPanel class. But additionally it has 2 new properties: VariableSizedWrapGrid.ColumnSpan and VariableSizedWrapGrid.RowSpan. The ColumnSpan property is used with the Orientation=Horizontal, and the RowSpan property with the vertical orientation. These properties contain an integer number as a value and if to set it higher than 1, the panel will multiply the space for the element.
Here is the example:

And that’s how it is displayed depending on the available size.
800px
if to narrow to 400px
and 300 px

There are two additional panels: CarouselPanel and WrapGrid. But the CarouselPanel class can’t be used outside the ComboBox class (it is used as the default ItemsPanelTemplate), and the WrapGrid class also should be situated inside a ItemsControl class. Maybe in the release version they will be available for using in any places, but so far we can use only 5 above described panels.